1. Field of the Invention
The present invention relates to an optical sampling measurement apparatus and an optical sampling measurement method of carrying out sampling of a light to be measured, using a nonlinear optical effect to measure a waveform or the like of the light to be measured.
2. Description of the Related Art
Recently, optical communication technology exponentially evolves and a light sampling measurement apparatus actively develops which measures a waveform or the like of optical pulse propagating through an optical fiber for use in an optical transmission path, with evolution of optical communication technology. Proposal is made about a light sampling measurement apparatus which receives the light to be measured and a pulsed sampling light having a (temporally) narrow width, by a nonlinear optical element and which generates a sum frequency light wave in the nonlinear optical element, as a kind of light sampling measurement apparatus. The light sampling measurement apparatus has a light receiving element such as an avalanche photodiode for receiving the generated sum frequency light wave.
Such a light sampling measurement apparatus compares a predetermined threshold level with a reception signal level of the light to be measured, in order to determine existence of signal pulse (“1”:mark) or nonexistence of signal pulse (“0”:space). It is important to accurately measure a signal to noise ratio (SNR) and a Q value (which will be collectively called Q value hereinafter), on measuring a communication quality and a waveform of light signal (light to be measured) in the optical communication.
Now, it will be assumed that μ1 represents an average of received signal levels each of which has “1” and σ1 represents a variance of received signal levels each of which has “1”. Furthermore, it will be assumed that μ0 represents an average of received signal levels each of which has “0” and σ0 represents a variance of received signal levels each of which has “0”. Q value is given by a following Equation (1).
                    Q        =                                            μ              1                        -                          μ              0                                                          σ              1                        +                          σ              0                                                          (        1        )            
By the way, it is necessary to accurately measure the average and the variance of signal levels of the light to be measured, as described above, in order to accurately measure the Q value of the light to be measured. However, the variance has noises which are generated in the light sampling measurement apparatus, on measuring the variance of light to be measured. There are a thermal noise and a shot noise each of which is generated in the light receiving element and there is an amplifier noise which is generated in an amplifier for amplifying an photoelectric current outputted from the light receiving element, as the noises generated in the optical sampling measurement apparatus.
More particularly, a power of the light (sum frequency light wave), which is inputted to the light receiving element, becomes very weak, in the optical sampling measurement apparatus using the nonlinear effect. For example, the power of light, which is inputted to the light receiving element, becomes equal to or less than 1/1000 of the power of light which is inputted to a light receiving element installed in an optical sampling measurement apparatus which does not use the nonlinear optical effect. As a result, the ratio of noise becomes great in comparison to the received light signal which is obtained by receiving the light to be measured. It is difficult to accurately measure the variance of signal levels of the light to be measured. Therefore, it is difficult to accurately measure the Q value of the light to be measured.